Automatic frequency control for oscillation systems



Nov. 161, 1937., A. WHEELER AUTOMATIC FREQUENCY CONTROL FOR OSCILLATION SYSTEMS Filed Jan. 2'5. v195e; sheets-sheet 1 Nv.r16, 1937. H. A. WHEELER# `AUTOMATIC FREQUENCY 'CONTROL FOR oscILLA'r'IoN sYTEMs F11-ed Jan. 25, 195e 2 sheeits-sheet 2 Patented Nov. 16, 1937 'PATENT OFFICE AUTOMATIC FREQUENCY CONTROL FOR OSCILLATION SYSTEMS Harold A. Wheeler. Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application January 25, 1936,. Serial No. 60,82

29 Claims. (Cl. Z50-720) This invention relates to the regulation of the resonant frequency of a tuned or tunable system and, while it is of general application, it is particularly suitable to the control of the frequency 5 of a local oscillator of a superheterodyne receiver so as to obtain and/or maintain the carrier frequency in an intermediate-frequency channel thereof substantially at the center frequency, or at least at the normal operating frel'quency, of such channel. This application is a continuation-in-part of my copending applica- .tion Serial No. 46,493, flied October 24, 1935.

v As is well understood, the intermediate-frequency channel of a superheterodyne receiver is a lchannel passing a given fixed intermediate frequency and at least one of its symmetrically related side bands of modulation. As is also well understood, reception with the greatestselectivity and fidelity of which such a receiver is capable is usually obtained, other things being equal, when thefrequency of the intermediatefrequency carrier wave produced by the receiver is located at the center or normal operating frequency of the intermediate-frequency channel.

In common practice, however, the relationship just described often does not obtain because of the fact thatthe oscillations actually produced by the local oscillator of such a receiver drift relative to the frequency which the oscil- 30 lator is intended to produce. This drift is particularly noticeable during each initial period of operation of the receiver in which'the various elements in the receiver are heating up to their steady operating temperatures.

35 tors, such as variations in the supply voltage, also contribute to oscillator drift. Oscillator drift is frequently of such magnitude as not only seriously to impair the fidelity of reception of a given signal but also to cause the complete fad- 40 ing out of such signal.

ceiver is generally required in either instance.

A second cause for the deviation of the intermediate-frequently carrier wave from the center or normal operating frequency of the intermedif 45 ate-frequency channel resides in the fact that the receive may be inaccurately tuned to the frequency of the received signal. As a result, the tunable local oscillator of such receiver produces oscillations of afrequency different from that which it produces when the receiver is tuned precisely to the frequency of the received signal.

Each of these two factors becomes of increasing importance as the signal frequencies are higher in the frequency spectrum. Thus, the same 5* percentage of frequency deviation incidental to Other fac- Returning of the reeither factor produces an actual frequency deviation at 10 megacycles which is ten times greater than that produced at 1 megacycle.

It is an object of this invention, therefore, to provide a simple and4 effective automatic con- 5 trol of the resonant frequency of a tuned circuit and, more particularly, of a local oscillator of a superheterodyne receiver.

More specifically, it 1s an object of the invention to provide an automatic frequency control 10 .for a local oscillator of a superheterodyne receiver effective to maintain the frequency of the intermediate-frequency carrier wave substantially at the center or normal operating frequency' of the intermediate-frequency channel. l5

It is another object of the invention to provide an automatic frequency control of' the type described which has one or more of the following characteristics: uniform effectiveness throughout the tunable range ofthe receiver; 20 equal effectiveness in compensating for tendencies -of the intermediate-frequency carrier wave to deviate veither above or below the center or normal operating frequency of the intermediate-frequency channel; independence from variations in amplitude of the intermediate-frequency carrier' wave; and independence from the carrier wave of the desired signal during periods of severe fading thereof. A further object of the invention is the provision in such an automatic frequency control system of means for indicating the sense, or. alternatively, both the sense and relative amount. by which the frequency subject to control tends to deviate from the desired normal frequency, or,`more specifically, by which the frequency oi.- the carrier wave in the intermediate-frequency channel tends to deviate from the center or normal operating frequency thereof. With such a feature embodied in a radio receiver, the user of the receiver may readiust its tuning to insure the maximum fidelity of reception and the maximum margin of automatic frequency control.

Other objects of the invention. will become apparent from a consideration of the following specification taken in conjunction with the accompanying drawings and the appended claims. The invention contemplates a tuned circuit, preferably electrically symmetrical with respect 5U to its output connections and including at least four impedance elements arranged in two parallel arms, each arm consisting of two seriallyrelated impedance elements', at least two of such impedance elements, one in each arm, being rea active and of opposite type, together with means individual to an impedance in each arm for adjusting in the same sense the effective inductance and capacitance values in the tuned vcircuit of the reactive elements of opposite type, thereby controlling the resonant frequency of the tuned circuit. Such control may be effected automatically by impressing a control voltage of such frequency. of a frequency related thereto in a predetermined manner, or of acontrolling frequency, upon two control elements the responses of which vary in opposite senses as the control the associated control element. The relative pro- Y portioning and disposition of the various elements a'nd circuits is' such as tocause the effective values .of the circuit reactances of the two arms to vary in the same sense with variations in the frequency, and in the opposite sense with variations in the amplitude, of the control voltage.

More specifically, when the invention is applied to a'superheterodyne receiver, the tuned circuit comprises the oscillation circuit of a local oscillator, and thel control is effected in response to deviations of the intermediate-frequency carrier wave from the center or normal operating frequency of theintermediate-frequency channel. `Such control may be effected by two auxiliary circuits sharply tuned respectively above and below the center or normal operating frequency of such channel and coupled thereto. The outputs of the auxiliary circuits are utilized to adjust the effectiveness of the frequency-determining reactance elements of the oscillation circuit. In the .embodiment illustrated hereinafter, the outputs of the auxiliary circuits are rectified and ,applied as bias voltages to the grids of vacuum tubes effectively shunting the respective reactan elements.

In accordance with a preferred embodiment of the invention, the local oscillator to which automatic frequencyccntrol is applied is the second, vor fixed frequency, oscillator of a double superheterodyne receiver, the frequency of the oscillator being controlled by the second intermediate-frequency carrier wave. With such an arrangement. the automatic frequency control is of uniform effectiveness regardless of the frequency to which the receiver is tuned.

In accordancewith another feature of the invention, the automatic frequency control is provided with a relatively long time constant so that it remains eifective independently of the carrier wave of the desired signal during short periods of severe fading thereof.. Moreover, this insures that the automatic frequencyicontrol, during a period of severe fading of the desired signal, does not attach itself to -an undesired signal. Further, in certain cases, the time constant of the automatic frequency control is subject to,adjustment by the user of a receiver, or automatically in ac- -7. cordance with the tuning thereof, so that the' the receiver to a desired signal and thereafter intime constant'.l is small during initial tuning of creases to its higher lvalue' during normal receptlon. In accordancewith still another feature of the invention, there is provided in connection with of adjustment required to retune the receiver. With the particularautomatic frequency control described herein, indication of the sense andthe degree of frequency deviations is vconveniently obtained by connecting a zero-center voltmeter across two series-connected resistances, the currents in which are in opposition and individually vcontrolled by the responses in the two sharply resonant circuits.

In the drawings, Fig. 1 is a circuit diagram',-

partly schematic, illustrating an embodimentof the invention as incorporated in a superheterodyne receiver; Fig. 2 is a schematic diagram of one type of indicating device represented in Fig.

' 1; Fig. 3 is an outline of another type of indieating device'represented in Fig. l; and Fig. 4`

vis a circuit diagram of a modifiedv -form of the control'. apparatus of the present invention shown in Fig1 1.

, vReferring now to Fig'. i of the drawings, the superheterodyne receiver there shown comprises a tunable radio-frequency channel I2 connected to the antenna l0 and ground Il. The radiofrequency channel i2 may or may not include amplifying means, as desired. Connected in oascade with theI radio-frequency channel i2 area mst frequency changer n, a first lntermediatefrequency channel Il, a second frequency changer Il', a second intermediatefrequency channel I6, a detector and automatic gain .control source l1. an audio-frequency amplifier I8, and a sound reproducer I9. All this apparatus may be cf conventional design'. The first frequency changer I3 is tunable and comprises a tunable oscillator and rst modulator, the tunable oscillator being tuned conjointly with the tuning of the radio-frequency channel I2 by a unicontrol mechanism indicated by the dotted line 22. The second vfrequency ichanger comprises a second modulator 23 and a nontunable" oscillator 24 the output frequency of which is subject to control within limits as described hereinafter. A "tunable frequency changer or oscillator, as used herein, is defined as one which is normally operable over a substantial range of frequency directly in response to adjustment by the user. An untunable frequency changer or oscillator, as used herein, is defined as one which is normally operable atv a frequency that is fixed in so far as any direct control of the frequencies by the user is concerned, but maynevertheless be adjustable within relatively -narrow limits for the purpose of regulating the frequency .output thereof. Either or both of the intermediate-frequency channels may include amplifying means. Preferably,y however, second intermediate-frequency channel I l is provided with amplifying means andis, as described hereinafter, more selective than the first intermediatefrequencychan'nel.- .The automatic amplification control bias potential, indicated as A. V.- C., may be applied to one or more of the radio-frequency channelA Il, the modulator of the first frequency changer Il,- the first intermediate-frequency I5 is non-tunable and channel i4, the second modulator Il, and the sectuned circuits 8| and 82 suitably energized from the second intermediate-frequency channel I8.

ond intermediate-frequency channel I8, as desired.

Considering first the operation of the receiver as thus far described, the desired received signal is selected (and also, in some instances, amplified) in radio-frequency channel I2 and converted by the first frequency changer Il into a signalmodulated first intermediate frequency. The'slgnal, as thus converted, is selected (and also, in some instances, amplified) in the first intermediate-frequency channel I4, converted by the second frequency changer I8 into a signal-modulated second intermediate frequency, selected (and also, in many instances, amplified) in the second intermediate-frequency channel I8, converted by detector I1 into the audio frequencies of modulation, amplified in the audio-frequency amplifier I8, and reproduced by the sound repro ducer I8. The amplification of the received signal\ is subject to automatic control by the-A. V. C. bias potentials, according to the manner well under- -stood in the art.

In orde'r to maintain the second intermediatecarrier frequency of the above described system at the center or normal operating frequency of the second intermediate-frequency channel I8, there is connected to suchvsecond intermediate-fre-A 'I'he two tunedcircuits 8| and 82 are sharply tuned, respectively, to frequencies below and above the normal operating frequency, -in this instance the center frequency, of the second intermediate-frequency channel and are connected, respectively, to rectiiiers 38 and I4, preferably of the diode vacuum tube type. The load circuits of the rectiiiers 38 and 34 comprise the parallel connected condensers and rresistors 88, 31 and 88, 88, respectively, preferably grounded at the rectifier cathodes, as shown.

If desired, additional resistors 4I "and 42 may be provided in series with resistors 81 and 88,.

respectively, and a zero-center voltmeter 43 connected thereacross,

The non-tunable oscillator I4 may be of any of certain conventional types. One preferred arrangement, however, is that illustrated, in which the oscillation circuit 881s effectively connected in the grid-cathode circuit of a vacuum tube 5|, withl the anode-cathode circuit containing an inductance 82 by which the anode circuit is tuned to a frequency above the. resonant frequency of oscillation circuit 88. The oscillator is provided with a grid choke 84 and coupling condenser 88, and with afeed-back coupling condenser 8l between the anode and grid circuits. Preferably, grid choke 84 is of such proportions as to tune the oscillator 24, other than the oscillation circuit 80, broadly to the resonant fre- `form of substantially pure negative resistance,

such as a dynatron, may alternatively be utilized in the oscillator 24 in connection with the oscillation circuit III.

They oscillation circuit of the non-tunable oscillator 24 comprises not less vthan four im pedance elements, of which at least two, in

opposite arms of the circuit, between the output connections 81, 88 thereof, are reactance elements of opposite type. In the arrangement shown, the oscillation circuit 58 comprises four lation circuit has a reactance which is not greater than a minor fraction of the total reactance in the corresponding leg of the oscillation circuit and, preferably, which is equal in magnitude to the reactance of the corresponding element of the opposite type. Specifically, the 'oscillation circuit 50 comprises two serially-related inductances L1, La and two serially-related condensers C1, Cz connected in parallel effectively between the grid and the grounded cathode of vacuum tube 8|. Inductance L2 is a minor fraction of the total oscillation circuit inductance and, at the normal resonant frequency of oscillation circuit SII, the capacitive reactance of condenser C2 is substantally equal to the inductive reactance of inductance L2.

'I'he oscillation circuit 80 is thus electrically symmetrical with respect to the output connections 51, 58, which are points of maximum voltage difference. The term electrically symmetrical, as applied to a tuned ci'rcuit, is used herein to describe a tuned circuit in which the impedances in the two parallel paths between the points of reference have, at the natural resonant frequency of the circuit, substantially equal values and opposite phase angles. In addition, the impedances of oscillation circuit 50 are so proportioned and arranged that point 88 (the junction of L1, Lz) is a vpoint in one arm of the circuit which electrically corresponds with the point 80 (the junction of C1, C2) in the other arm of the circuit. The term electrically corresponding points in the respective `arms of a tuned circuit, as used herein, is defined as two points` to which the impedances from a iunction of the two arms have, at the natural resonant frequency of the circuit, substantially equal values and opposite phase angles.

In order to control the frequency of the oscillator 24 by ymeansvof the frequency responsive device 28, there is provided the frequency control apparatus 21 comprising'a vacuum tube 83, effectively shunted across inductance L: through a blocking condenser 85, and a vacuum tube 84, eiIectively shunted across condenser C2 through blocking condenser 88. A source of operating voltage, such as a' battery 18, for the tubes 83 and 84 is connected to their anode circuits through Vchoke coils 81 and 88, respectively. 'I'he grids of the tubes 8l and 84 are connected tu the appropriate terminals of resistors 81 and I8, respectively, and are'` thus biased with voltages varying with the responses of the sharply tuned circuits 8l and 82, respectively. The grid circuits of tubes 88 and 64 may be provided with filter sections 1|, 13, and 12, 14, respectively, each comprising a blocking resistor and by-pass condenser.

In certain cases, filter sections 1Ia, 18a and 12a, 14a, in addition to and associated with filter sections 1I, 18 and 12, 14, respectively, may be provided in the grid circuits of the tubes 88 and 84 so lthat for each grid circuit the two corresponding filter sections are in cascade, together unicontrol mechanismfl with two switches 85 and 86 by means of which the additional filter sections. normally effective, may be short-circuited and thus rendered ineffective. Switches 85 and BB are preferably operated conjointly by unicontrol' mechanism indicated by the dotted line 81. Also, the unicontrol tuning mechanism 22 is preferably interlocked with the unicontrol mechanism l1 so that, whenever,the receiver is tuned, thev switches 85 and 86 are actuated toArender the 'additional lter sections ineffective. This interlock is schematically indicated by an arrangement in which the tuning knob 88 must be pushed inwardly against the action of a spring 94 to engage the tuning unicontrol mechanismff. the inward `motion f' of the knob actuating switches I5 and 8B through unicontrol mechanism 81. Other Ainterlocldng arrangements may, of course, be employed; for' example, switches 85 and B8 may be actuated by the us'ers hand in proximity to or touching a tuning knob directly connected to the tuning If desired, series-connected resistors and 1B may be'connected in the anode circuits of `tubes 63 and B4, between high-frequency chokes I1 and il, respectively,with source 18 connected to the junction of such resistors and with a, zerocenter direct-current meter 11 connected across the terminals thereof. Alternatively, connected resistors 18 and 8U may be connected in the cathode circuits of tubes B3 and 64, between the cathodes of such tubes, with the junction of the resistors grounded and with a zerocenter direct-current meter 8l connected across the terminals thereof., In this latter case, re-

-, sistors 19 and 80 `are preferably by-passed by condensers B3 and 84, respectively. Also, if desired, indicating lamps 69 and v10 may be connected in place of, or in shunt or seriesv with, resistors 15 and 18, respectively.

In considering' the operation. of the abovedescribed automatic frequency control system, it 1 is first assumed thatthe radio-,frequency channel I2 -is tuned precisely to the frequency of the desired signal, that'the tunable oscillator ofthe first frequency changer I3 is tuned precisely to,

and is producing oscillations precisely of, such a frequency that the frequency of the first intermediate-frequency carrier wave is located at the .center frequency of the first intermediate-frequency channel i4, and that the non-tunable oscillator 24 is producing oscillations of such a fr'equency that the frequency of the second intermediate-frequency carrier wave is located at the center frequency of the second intermediate-frequency channel I6. To simplify the description of the basic operation'of this receiver, the functioning of the previously mentioned optional proabove the resonant frequency of the sharply due to the shunted inductanceh is equal to they visions will, for the moment. be neglected.

Under the conditions thus assumed, the frequency of the second intermediate-frequency carrier. wave is 'as'much below the resonant frequencyV of the sharply tuned circuit'llas it is tuned circuit ll. The responses of .these tuned circuits are thus equal in magnitude. with the result that equal voltages appear across resistors net capacitive reactance in the oscillation circuit due to the shunted condenser Ca. the oscillator I4 acaaisc seriesmediate-frequency carrier wave falls below the center frequency of the channel I4, assuming that the frequency of the tunable oscillator is above the frequency of the received signal.- lTo an` equal extent, the frequency of the second intermediate-frequency carrier wave tends to4 fall below` the center frequency ofthe channel I6, assuming that the frequency of the non-tunable oscillator 24 is below the frequency of the second intermediate-frequency carrier wave.

As the frequency of the second intermediatefrequency carrier wave becomes nearer to the resonance frequency of tuned circuit 3| further from the resonance frequency of tuned circuit Y32, the responses of the two circuits -become unbalanced correspondingly and unequal negative potentials are developedl across resistors 31 and 38. 'I'he negative bias potentials applied to the control grids of vacuum tubes B3 and 84 are similarly unbalancedjthat applied to the control grid of tube BI assuming the larger nega.- tive value. The resistance'of the shunt across 'La becomes, therefore, greater, and the resistance of the shunt across C: becomes lessthan their respective resistances under the conditions initially assumed. As a `consequence; the effective inductance of L1 and Lz in series and the effecvtive capacitance of C1; and C2 in series are both increased, and, likewise, the effective-inductance and u and the effective capacitance of the oscillation circuit 5l are correspondingly increased. Oscillator24 thus operates at a correspondingly decreased frequency. By suitable choice of circuit constants this decrease in frequency of the nontunable oscillator 24 may be made very nearly to approximate and compensate for the decrease in frequency of the second intermediate-frequency carrier wave which, in the absence of the compensating action of this invention, would occur as a result of the assumed drift of the tunable oscillator. As a result.v the second intermediatefrequency carrier wave is maintained very nearly exactly Aat the center frequencyof channel IB. Obviously, in case' the frequency of the second intermediate-frequency carrier wave should increase above the center frequency of channel IB, the system would operate in a similar manner but reverse sense. o

It will be apparent that. in the event the frequency deviation of the carrier wave in channel 'I8 is lproduced by drifting of the non-tunable oscillator 24, or by mistuning of the receiver, or

any other cause, the frequency responsive device 26 and the frequency control apparatus 21 will -be effective to compensate for such deviation in a similar manner. 'I'he final result of this control ofthe frequency .of the non-tunable oscillator I4 is that` the second intermediate-frequencyl carrier wave' is maintained very nearly at its normal frequency, i. e.. the center frequency of the second intermediate-frequency channel, re-y gardlessof all frequencydisturbing causes.

In selecting the time constants of thel filter sections 11, 1l and 1l. 14 (neglecting the adcii-v Il tional lter sections). consideration should pref, the frequency of such'carrier wave deviates someerably be given to the circumstances under which the "receiver Vis used. If the receiver is iised for the reception of signals the carrier waves of which are subject to severe fading, the

time constants o f these two filter sections are lis preferably lequal and suiliciently great to prevent the biases on tubes 83 and 84V from changing f .such as of the order of one second or less. If,

in such case, the receiver is provided with auto- `matic amplication control (A. V. C.). the time constant of the automatic frequency control- (provided by the time constants of the filter sections) is preferably greaterat least doublethe time constant of the automatic amplification control. A common time constant for A. V. C.

, being of the order of one-twentieth of a second,

the time `constant of the automatic frequency control may, when the signal carrier wave is not subject to severe fading. be of the order of onetenth to one-fifth of a second.

When the time constant .of the automatic frequency control is small, the control tends to assist the tuning of the receiver to a desired signal,by compensating for inaccurate tuning by the user thereof. When the time constant of the automatic frequency control is large, it is highly desirable that the user initially tune the receiver to the desired signal with accuracy, thus assuring the user full benefit of the automatic frequency control during the continued reception of such signal.

In certain cases, provision may be made by which the time constant of the automatic frequency control is subject to adjustment by the user of the receiver. In such cases, for example, the initial tuning of the receiver to a desired signal may be made utilizing a small time conf stant in the automatic frequency control. with subsequent reception of such signal utilizing a` large time constant in the automatic frequency control. This provision is embodied in the previously mentioned additional nltex' sections lla, 13a and 12a. 'Ila and the equipment related thereto. The switcheslland 88 are normally in the positions illustrated and render the additional filter sections effective, thereby imparting a large time constant tol the automatic frequency control. Preliminary to tuning the receiver by the tuning knob ll, the user automatically 'causes switches 8B and Q6 to be actuated to render the additional filter sections ineffective, thereby reducing the time constant of the automatic frequencycontrol to a smaller value. During the tuning of the receiver the additionalfllter sections are maintained ineffective andthey resume their effectiveness only when the user ceasesto tune the receiver.

It is to be noted that. inasmuch as( the carrier wave in the first intermediate-frequency channel Il is not maintained at a fixed frequency. the filter of channel il is preferably rather broadly rierwave and its side bands of modulation as what from the center frequency of channel Il. In contrast, the filter Vof channel I6 may be, and preferably is, very selective or critical, since the frequency of the carrier wave therein is maintained nearly constant. lIf desired, channel I8 may be provided with any well-known arrangement providing variable selectivity. l

It is also to be noted that the automatic frequency control of the second intermediatel-frequency carrier wave as described above is independent of its amplitude. Thus, if the amplitude of the carrier Vwave increases or decreases from a given valueythe negative bias potentials applied to the control grids of tubes B3 and 64 arel varied simultaneously in the same sense and to x a similar extent, and the effective inductance of La and the effective capacitance of C: are correspondinglyA varied but in opposite senses. The

lresulting changes in the effective reactances of the arms of the oscillation circuit are thus equal and opposite and the resonant frequency of oscillation circuit 50, therefore, is substantially unchanged. In accordance with a .feature of the invention,

the automatic frequency control circuits may be designed to operate .either moreor less critically,

as desired: More particularly, three different elements upon which the control voltages are impressed. initial bias voltages are provided which are substantially less than the value required to cut off the plate currents of the respective tubes, and the variable control biases are of such magnitude and polarity that the resultant grid bias is at all'times less than the cut-off value. Here, therefore, the operations! the frequency control is critical and responds to all variations in the control voltages from their normal values. j

In Class B operation, an initial ynegative biasing voltage is utilized which is approximately that required to cut off the plate current; and in Class C operation an initial negative biasing voltage is employed which is substantially greater than the cut-off value. 'I'he incremental or con trol biasing voltages in these last two instances are applied positively to the control elements so as to reduce the resultant grid bias to values less than the cut-off value. In Class B operation, the initial biasvoltage is such that the eiIect of a small incremental or control voltage is much less than in Class A. In Class C operation. the initial bias voltage is excessive, so that the incremental or control voltages are quite ineffec- It will be apparent that the embodiment of the invention shown in Fig. 1 may be considered as eecting Class A operation, in accordance with the meaning of this term as hereinabove dened. In Fig. 4 there is\illustrated a modified form of the control apparatus of the invention whereby either Class B or C operation may be obtained. Excepting for Vthe differences hereinafter specifically described. the circuits shown in Fig. 4 are identical with the corresponding circuits of Fig. l. Corresponding parts are given the same reference numerals in the two figures and the.

detailed description of such parts is not repeated.

While, in Fig. 4, only the frequency responsive` device 26a and the frequency control apparatus 21a, which correspond to the device 28 and apparatus 2li, respectively, are shown, -it will be understood that these circuits are adapted to be included in a receiver in substantially the same manner vas the corresponding circuits of Fig. 1.

Thus, the leads from the second intermediatefrequency channel ICSl will be connected to the frequency responsive device by way of the conf nections 92 and the leads from the oscillation circuit d Will be connected to the frequency control apparatus by way of the connections 93, these connections being indicated in both figures. In the modified form of the invention, a biasing battery 94 is provided inl the common lead of the cathode circuits of the tubes 63 and 64 and, as shown in the drawings, may be connected between ground and the iunction of the resistors 20 19 and 8E. This battery serves to apply a substantially fixed initial biasing voltage, above the relatively small initial bias voltage developed across the resistors 19 and 8B by the space currents of the tubes S3 and 64, respectively, as in the embodiment of Fig. 1. The voltage supplied by the battery 94 may be made sufficient to cut od nearly or completely the plate currents of the tubes d2 and dit. With this condition. it is necessary that the incremental or control bias voltages supplied by the frequency responsive device 26a be appliedV to the control grids oi the tubes dfi and te in a positive sense in order, that the resultant negative bias voltageapplied to each v tube be reduced from the cut-oid value. For 35 this purpose, two further changes are made in the circuit of Fig. 4. over that of Fig. l. The circuit connections to each of the rectiflers t3 and 34 are reversed. The bias connections between the' device 26a and the apparatus 21a are then interchanged so that the rectifier 33, in this embodiment, controls the tube 64 and the rectifier 34 controls the tube 63.

These modications result in substantially the same frequency controlling action as described with reference to Fig. .1, but, instead of obtaininglass A control, as above defined, either Class IB or Class C control may be obtained. That is, this embodiment hasy the additional advantage of less critical operating conditions. More particularly, the frequency control will not be effective on weak signals, as for instance signals which are too weak to operate the automatic volume control; there will be less response to small accidental frequency variations and the constants of the con- 55 trol circuits need not be critically proportioned.

If the initial bias voltages on the tubes B3' and 64 are made approximately that required to out off the plate current, ClassB operation will be obtained, While if' the initial bias voltage is suba() stantially-'greater than that required to cut off the plate current. still less criticalfor Class C. op-

eration is obtained.

An inherent characteristic of the arrangements above described is that the frequency control is substantially unaffected by yvoltage uctuations. such as those of the source supplying operating voltages for the frequency controlling tubes GA3 and 84. This condition obtains because such voltage fluctuations are applied similarly to both 'y0y tubes and tend to cause substantially equal and opposite effects on the frequency o f the oscillator.

A particular advantage yof this property of the'`- circuit resides in the fact that it is not sensitive to hum ripple components on the power voltages 75 and, likewise, is not sensitive to audio-frequencyV coupling through the direct-current circuits. The former would be evidenced byv hum in the receiver output when the receiver was slightly detuned, so as to convert any hum modulation to modulation of the carrier. The latter would be evidenced as4 motor boating when the receiver was slightly detuned on either side.A Both of these effects would be highly detrimental if they u were not. obviated, as above described, by the present invention. l.

lConsidering now the previously mentioned optional provisions, one or more of such provisions /are preferably utilized toprovide indicating means to aid in accurate tuning of a receiver incorporating automatic frequency control. Although precision tuning is not necessary in order to obtain satisfactory reception with such a receiver, it is sometimes advantageous and aids in obtaining the highest quality reception possible and in avoiding loss or impairment of the received signal during the operation of the receiver.

Illustrative of such optional indicating means -is the meter 43, shown in greater detail in Fig.

2, 'together with its associated resistors 4l and The direction and extent of the deflection of pointer ii@ of such meter from its zero-center position is dependent upon the` algebraic sum of the voltages across the two resistors 4I and 42. These l voltages are severally dependent upon the magnitudes of currents' rectified by the respective tubes `33 and 3Q, which, in turn, are dependent upon themagnitudes of the responses of the respective tuned circuits 3l and 32. When the frequency of the second intermediate-frequency carrier wave is at the center frequency of the channel I6, the responses of tuned circuits 3| and 32 are equal in magnitude, `so that the voltages across resistors 4i and 42 areequal and opposite and the pointer 81 remains at its zerocenter position, as illustrated in Figs. 1 and 2. WhenH the frequency of the second intermediatefrequency carrier wave tends to deviate from the center frequency of the channel i6, the voltages across resistors 4I and 42 become unbalanced in sense and extent corresponding to such frequency deviation, which deviation is indicated by the de` vvice 43. As the receiver is retuned to bring the frequency of the second intermediate-frequency carrier wave`to the center frequency of the '.channel L8, independently of the automatic frequency control, the voltages across resistors 4l and 42 are rebalanced so that the pointer 89 gradually returns to its zero-center position.

Each of the lndicatingmeans comprising zerocenter meter 11 and resistors 15 and 16, and that comprising zero-center meter 8| and the resisters 19 and 80, is similar in principle and operation to the indicating means comprising meter 43 and resistors 4i and 42, the only difference rel siding -in the fact that the currents flowing in the resistors of the former two indicating means are the space currents of the tubes 63 and 84 in stead of the currents 'rectified by the tubes 33 and 34. Inasmuch as the space currents of tubes Y B3 and 64 are generally of greater magnitude than the currents rectified bythe tubes 33 and L34, it may be desirable in many cases to utilize shunted around reslstances 18 and BIJ by means of .thecondensers 83 and'84. An alternative form ofv indicating means to aid in tuning the receiver accurately to the frequency of the desired signal wave comprises the two indicating lamps SQ and 10, connected across resistors l5 and 16, respectively. When the fre-- quency of the second intermediate-frequency carrier wave is located at the center frequency of the channel i6, the voltage across the two lamps are equal in magnitude, with the result that they. operate with equal brilliance. When the frequency of the second intermediate-frequency carrier wave tends to deviate from the center frequency of the channel IS, the. voltages across the lamps become unbalanced in sense and to an extent corresponding to such deviation, which is indicated by the relative brilliance .of the lamps.

A simple and convenient arrangement for enabling the brillianceof the lamps 69 and lil to be comparedis illustrated in Fig. 3. A suitable opaque panel 90, such as the panel of the receiver, is provided with a translucent window 8i. behind which are mounted the two lamps 69 and l0 preferably with a. light screen $5 extending therebetween.

It is to be understood that the indicating means comprising lamps 69 and 10 may be 4einployed in lieu of any-of the indicating means which include meters 43, 11 or Il. Each of the illustrated indicating means indicates both the extent and the sense of the frequency deviation. If desired. however. suitable indicating means, indicating only the sense of the frequency devi. ations of the second intermediate-frequency carrier wave, may be substituted for any or all of the indicating means, where such ane satisfactory. It will be apparent that the indicating means herein described' may be used with receivers in which automatic frequency control is obtained by apparatus or methods other than herein described.

It is to be noted that, while tubes il and 64 of the frequency control apparatus 2l are shown and described as having their space paths effectively shunted, respectively, across inductance Le and capacitance Cz. in certain cases tubes 83 and 64 may be associated with the two reactance arms of the oscillation circuit Il in other manners. The basic characteristic of such association is that the effective value of each of the two reactance arms is subject to individual control by the tube associated therewith.

Attention is directed to the fact that, although the oscillator in the superheterodyne receiver which is controlled by device 28 and frequency control apparatus 21 is preferably a non-tunable oscillator, such as the second oscillator of a triple detection superheterodyne, any other oscillator of a superheterodyne receiver may be so controlled, including the tunable oscillator. Thus, automatic frequency control of the .tunable oscillator of a double detection superheterodvne receiver may be employed if the resulting sacrifice in the uniformity of effectiveness in the automatic frequency control throughout the tunable range of .the receiver is not objectionable.

While the invention has been described in connection with a superheterodyne receiver 'and as concerned with maintaining an intermediate frequency carrier wave in such a receiver at .a constant frequency, the invention is of more general application and is directed broadly to controlling and/or varying the resonant frequency of a tuned system without physical alteration of the reactance elements, perse, of such a system.

While I have described what I at present corisider the preferred embodiment of my invention, it will be obvious to those skilled in the art that various changes and modiilcationsmay be made therein without departing from my invention, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What is claimed is:

1. The method of selective reception of signalmodulated carrier waves which includes: selectively tuning to' any desired one of a plurality of modulated signal waves; generating and combining therewith a wave ofI approximately con stant but incidentally variable frequency separation from the carrier frequency thereof to produce a modulated carrier wave of first intermediate frequency; broadly selecting the carrier wave of first intermediate frequency corresponding to said desired signal wave to permit normal deviations in the frequency 'of saidcarrier wave without substantial attenuation of the modulation frequencies thereof generating and combining `avith said selected4 first intermediate-frequency carrier wave a second wave of substantially constant frequency to produce a modulated carrier wave 'of second intermediate frequency; modifying the frequency of said second generated wave within a range fof a small .per cent of its mean value in accordance withmnd responsive to.' predetermined deviations of the carrier frequency of said second intermediate-frequency wave. from its normal frequency to restore it thereto; sharply selecting the carrier hwave of second intermediate frequency corresponding to said desired signal wave:- and thereafter converting and utilizing said modulated carrier wave of second intermediate frequency. 'L

2. A tunable radio receiver comprising: a tunable flrst frequency changer for converting a selectedslgnal wave into a firstmodulatcd intermediate-frequency carrier wave of approximately constant frequency: broadly selective means for selecting said rst intermediate-frequency carrier wave while permitting normal deviations in the frequency thereof without substantial attenuation of the modulation frequencies thereof an untunable second frequency changer for converting said selected first intermediate-frequency carrier wave into a second modulated intermediate-frequency carrier wave subject to minor deviations from a given normal frequency; means responsive to said deviations of said second intermediate-frequency carrier wave for modifying the tuning of said second frequency changer to restore the frequency of said second intermediate.- frequency carrierwave to said given frequency; sharply selective means for selecting said second intermediate-frequency carrier wave: and means for converting. and utilizing said selected second modulated intermediate-frequency carrier wave.

3. A tunable superheterodyne receiver comprising; a tunable frequency changer for converting a selected signal wave into a rst modulated intermediate-frequency carrier wave of approximately constant frequency; a first intermediate-frequencychannel for broadly selecting said first intermediate-frequency carrier wave while permitting normal deviations in the frefrequency carrier wave 4subject to minor 4deviations from a given normalfrequency: asecond f5,

intermediate-frequency channel for sharply selecting andamplifying said carrier waves of second intermediate frequency, said second channel having a center frequency equal to said normal frequency; a device responsive to deviations in frequency of said second intermediate-irequen'cy carrier wave from said center frequency; and means controlled by said device for controlling the frequency of said untunable frequency changer to maintain the frequency of said second intermediate-frequency carrier Wave at said center frequency.

4. In a tuned circuit having an inductive arm and a capacitive arm eectively in parallel, means for controlling the resonant frequency ol such circuit comprising a pair of conductive paths individually eectively connected, respectively, across at least a portion of said inductive arm and across atleast a portion of said capacitive arm, and means for adjusting the resistances of said paths in opposite senses to alter simultane.

ously in the same sense the effective inductance and capacitance of said circuit.

5. In combination; a tuned circuit comprising two arms effectively in parallel each including at least two impedance elements, said arms including, respectively, reactance elements of opposite type: and means for controlling the reso.

nant frequency of said circuit without varying the physical values of any of said elements, compris-- ing a pair of conductive paths individually effectively connected, respectively, across a reactance element of one of said arms and across a reactance element. of the other of said arms, and means for adjusting the resistances of said paths to like extent but in opposite senses to alter simultaneously in the same sense the effective inductance and capacitance of said circuit.

6. In combination; a tuned circuit comprising two arms each including a plural and equal numi ber of impedance elements of which at least two are reactance elements oi opposite type, one in each of said arms, said impedance elements being proportioned and disposed so that said circuit is electrically symmetrical with respect to its points of maximum voltage difference; and means for controlling the resonant frequency of said cir- .cuit without varying the-physical values of any of said impedance elements, comprising a conductive path connected in shunt with a portion of each of said arms including one of said reactance elements, corresponding terminals of said paths being electrically corresponding points of said arms, and means for adjusting the resistances of said paths in opposite senses to alter simultaneously in the same sense the effective inductance and capacitance of said circuit.

'1. In combination; a tuned circuit containing at least two inductance and at least two capaci- 50 tance elements disposed andproportioned so that said circuit is electrically symmetrical with respoot to its points of maximum voltage difference. across which points the output of said circuit is derived; and means for controlling the resonant frequency of said circuit, comprising, two vacuum tubes having their space paths effectively connected respectively across only one inductance element and only one capacitance element of said resonant circuit, corresponding terminals of said one inductance and said one capacitance elements being electrically corresponding points of said arms, and means for controlling the conductance of said tubes in opposite senses to alter simultaneously in the same sense the eiective inductance end capacitance of said circuit.

8. In combination; a tuned circuit comprising in parallel. two serially-connected inductance elements and two serially-connected capacitance elements; and means for controlling the resonant frequency of such circuit without varying the actual physical value of any of said elements, comprising two variable resistance paths, one effectively connected across one of seid inductance elements and the other effectively connected across one of said capacitance elements, said paths normally having substantially the same resistance, and means for varying the resistances of said paths in opposite senses but to substantially like extents to alter simultaneously in the same sense the effective inducta'nce and capacitance of said circuit.

9. In combination; a tuned circuit comprising in parallel, two serially-connected inductance elements of substantially different inductance and two serially-connected capacitance elements of substantially different capacitance, the inductance velement of smaller inductance and the capacitanceelement of larger capacitance being disposed in corresponding portions of the two parallel arms; and means for controlling the resonant frequency of such circuit without varying the actual physical value of any of said elements, comprising two conductive paths individually connected across said inductance element cf smaller inductance and said capacitance element of larger capacitance, and means for adjusting the resistances of said paths in opposite senses to alter simultaneously in the same sense the effective inductance and capacitance of said circuit.

10. In combination; a tuned circuit having two terminals of which one is effectively grounded, and containing, in parallel between said terminais, twc series-connected inductance elements and two series-connected capacitance elements, the inductance and capacitance elements with one terminal thereof effectively grounded each having at the normal resonant frequency of said circuit a reactance which is substantially equal in magnitude to the reactance of the other and which is a. minor fraction f the total reactance of like kind in said circuit; Vand means for controlling the resonant frequency of said circuit, comprising, two vacuum tubes having their space paths eiectivelyconnected respectively across said inductance element and said capacitance element that have one terminal thereof eectlvely grounded, and means for controlling the conductances of said tubes in opposite senses to alter simultaneously in the same sense the effective inductance and capacitance of said circuit.

1l. In combination, a tuned circuit having inductive and capacitive arms, and means for varyving the resonant frequency thereof over a relatively small fraction of its normal resonant frequency in response to relatively small variations in frequency of a control wave from a given frequency and substantially independently of the amplitude of such control wave, comprising; two

control elements energized with said control wave, the responses of said elements varying in opposite senses over said relatively small frequencyvaristions of said control wave; and a. pair of control means individually responsive'to said-control elements and individually cooperating with the arms' inparallel, and means for controlling the oscil-- arms including not less than four impedance elements of which at least two are reactance elej ments of opposite type disposed in the respective 'arms of said circuit. and means for varying `the resonant frequency of said tuned circuit in response to relatively small variations of a control wave from a given frequency and substantially independently of the amplitude of such control wave, comprising; two control elements energized with said control wave, the responses of said elementsvarying in opposite senses over said relatively small frequency variation of said control wave; and a pair of independent similar control-circuits connected respectively from said control elements to the inductive and capacitive reactance arms of said tuned circuit, each of said control circuits including a biascontrolled vacuum tube having at least three electrodes'of which two are effectively .connected across said reactance element of the associated arm and including means for deriving a bias voltage the magnitude of which varies in accordance with the response of Athe associated control element, and for applying saidderived bias voltage'to the associated vacuum tube, said control circuit means and said bias deriving and applying means being relatively proportioned to vary the effective circuit values of the two reactance arms of said tuned circuit t in the same sense with frequency, and in oppo- 'site senses with amplitude, of said control wave.

13. In combination with ya tuned circuit having inductance and capacitance elements and normally resonant. at a predetermined frequency, means for controlling the resonant frequency of such'circuit without varying the physical'values of any of said elements, comprising: conductive paths eifectively in shunt respectively to an Ainductance element and a capacitance element of said circuit; a pair of auxiliary tuned circuits; t

tance in said tuned circuit in accordance with-i the responses of said auxiliary tuned. circuits.

14. In combination with a tuned circuit having 4inductance and capacitance elements and normally resonant ata predetermined frequency,

means for controlling the resonant frequency of said circuit without varying the physicai values of any of said elements, comprising: two vacuum tubes, each with a control-grid.- having their space paths effectively in shunt respectively to an inductanc'e element and a capacitance element of said circuits pair of auxiliary tuned circuits; meansfor energizing said auxiliary tuned circuits with a frequency having a flxed relationto the resonant frequency of said tuned circuit, .saidauxiliarytunedcircuits beingsharply resonant. respectively, at frequencies Just above and .HCI )mt below the normal value of said .related frequency; means for deriving rectified bles povarying in accordance with the responses es mammary tuned circuits: ancm'esns for L said potentials to the controlgrids of L 70 said tubes withthe same polarity whereby both.

the effective capacitance and effective induct'ance lation frequency thereof comprising: a pair of conductive paths individually connected, respectively. acrosslat least a portion of said inductive arm and across at least a portion of said capacitive arm of said circuit; and means dependent on the oscillation frequencyof said generator for controlling the resistances of said paths in opposite senses to altersimultaneously in the same sense the eifective inductance and capacitance oftively connected in shunt with an inductance element anda capacitance element of said circuit; andmeans responsive to the frequency of the output of said oscillation generator for varying the conductances of said paths in substantially equal amounts but in oppositesenses and responsive to the magnitude-of the output of said oscillation generator for varyingv the co n.

ductances of said shunts in substantially equal amountsbut in the same sense, whereby only variations in the frequency of the output of said oscillation generator inuenceth'e frequency produced by said oscillation generator. j i,

17'. In a frequency-changingV system in which a first selected input frequency and a second selected input frequency are ,arranged to produce a third frequency bearing a preqzleterrnined relation to said two input frequencies, and in which thev second input frequency is derived from an oscillation generator having inductance andcapacitance elements in\a frequency-determining Acircuit therefor; means, for maintainingV said third frequency at', ai preddermin'ed frequency' not`' withstanding incidenta minor variations in frequency of either or bot output of said system. the responses of .said controi elements varying'in opposite senses with vari- 4 of said input frequencies-I from their respective selected frequencies, comprising; two control elements energized with the ation of said output from said predetermined ire quencyiand means for controlling in the same `.sense the eective Acapacitance in the .frequencylected signal Vwave into a first modulated 'intermediate-frequency carrier waveof approximately .constant frequency:- anuntunabie second frequency changer for Vconverting said first intermediate-frequency carrier wave into a second modulated intermediate-frequency carrier wave subject to minor deviations'from a given 4normal frequency, said secondxfrequency changer including an oscillation generator having a ire- Iquency-determining circuit having an inductive arm and a capacitive. arm effectively in parallel;

'.ajpair'offconductive paths individually effec-` tively respectiveiy. acroat least a as to modify the action of said second frequency changer to restore the frequency of said second intermediate-frequency carrier wave to said given frequency; and means for converting and utilizing said second modulated intermediate-frequency carrier wave.

` 19. In a superheterodyne radio receiver: tunable means for selecting a desired signal wave subject to'severe fading and converting it to an intermediate-frequency carrier wave subject to minor deviations from a given normal frequency;r a critically selective intermediate-frequency filter control exercised thereby; time-delay means in said control means affecting both` said control means and said indicating means and of vsuiiierate said control means; and means operable cient magnitude to continue vthe control exercised by the control means during those periods of fading in which the carrier wave of the desired signal is of insuilicient magnitude to opwhen tuning said receiver for reducing to a smaller order of magnitude the time delay provided by said delay means, thereby to facilitate the tuning of said receiver with the assistance of said control means and said indicating means.

20. The methodvof selective reception of signalmodulated carrier waves which includes: selecting the desired modulated signal wave generating and combining therewith a wave of approximately constant but incidentally variable frequency separation from the carrier frequency thereof to produce a, modulated carrier wave of first intermediate frequency; selecting the carrier wave of first intermediate frequency corresponding to said desired signal wave; generating and combining with said selected rst intermediatefrequency carrier wave a second wave of substantially constant frequency to produce a modulated carrier kwave of second intermediate frequency; modifying the frequency of said second generated wave within a range of a Vsmall per cent of its mean value in accordance with, and responsive to, deviations in frequency above a predetermined magnitude, of the carrier of said second intermediate-frequency wave from its normal frequency to restore it thereto; and thereafter converting and utilizing said modulated carrier wave of second-intermediate frequency.

21. A tunable radio receiver comprising; a tunable nrst frequency changer for converting a-selected signal wave into a first modulated intermediate-frequency carrier wave of approximately constant frequency; an untunable second frequency changer for converting said rst intermediate-frequency carrier wave into a second modulated intermediate-frequency carrier wave subect to minor deviations from a given normal frequency; means responsive to said deviations in frequency, above a predetermined magnitude,

`arz'n and a capacitive arm effectively in parallel,

means for controlling the resonant frequency of such circuit comprising a pair of conductive paths individually effectively connected, respectively, across at least a portion of said inductive arm and across at least a portion ofsaid capacitive arm, and means for selectively adjusting the resistance of either of said paths in the same sense selectively to alter in opposite senses the effective inductance or capacitance of said circuit.

23. In combination; a tuned circuit containing at least two inductance and at least two capacitance elements disposed and proportioned so that said circuit is electrically symmetrical with respect .to its points of maximum voltage difference, across which points the output of said circuit is derived and means for controlling the resonant frequency of said circuit, comprising, two vacuum tubes having their space paths effectivelyv connected respectively across only one inductance element and only one capacitance element of said resonant circuit, corresponding terminals of said one inductance andsaid one capacitance elements being electrically corre` sponding points of said arms, and means for selectively controllingthe conductance of either of said tubes in the same sense selectively to alter in opposite senses the effective Vinductance or capacitance of said circuit.

24. In combination; a tuned circuit having two terminals of which one is effectively' grounded,

and containing, in parallel between said terminals, two series-connected inductance elements and two series-connected capacitance elements,'

the inductance and capacitance elements with one terminal thereof effectively grounded each having at the normal resonant frequency of said the conductance of either of said tubes `in the same sense thereby selectively to alter in the opposite senses the effective inductance or capacitance, of said circuit.

25.- In combination; a tuned circuitihaving two terminals of which one is effectively grounded,

and containing, in parallel between said terminals, two series-connected inductance elements and two series-connected capacitance elements, the inductance and capacitance elements with one terminal thereof effectively grounded each having at the normal resonant frequency of said' circuit` a reactance which is substantially equal in magnitude to the reactance of the other and trolling the resonant frequency of said circuit, comprising, two vacuum ltubes having their space paths effectively connected respectively across said inductance element and said capacitance element that have one terminal thereof effectively grounded, means for negatively blasing vsaid tubes at least to approximately plate current cutoff, and means for selectively applying in a positive sense a variable control bias voltage to either of said tubes for controlling their conductances in the same sense thereby .selectively to alter in the opposite senses the which is a minor fraction of the total reactance of like kind in said circuit; and means for cony respectively, across at least a 4portion of saidv effective inductance or capacitance of said circuit lwhen said variable bias voltages exceed a predetermined value sumcient to overcome the cut-oil effect of saidv tlxed bias voltage.

26.` In combination, an oscillation generator including a frequency-determining circuit hav-v ing an inductive arm and al capacitive arm 4 eiectively in parallel, and means for controlling the oscillation frequency thereof comprising:y a pair o! conductive paths individually connected.

inductive armand across, at least a portion of said capacitive arm` of said circuit: and means 'dependent on the oscillation frequency of said generator for selectively controlling the resistance of either of said paths in the same senses thereby selectively to alter `in the opposite senses the effective inductance or capacitance oi said circuit. v

27. In combination, an oscillation generator for generating oscillations, means ior controlling the frequency of said oscillations comprising two vacuum tubes so connected in circuit with said. generator that similar control oi said tubes pro-v duces equal and opposite enects on the frequency oi said oscillations. and means for simultaneously controlling said tubes in equal and opposite senses `to produce a cumulative 'shift of the frequency of said-oscillations.

28. In combination, .an oscillation generator including a plurality of reactance elements for generating oscillations and means for controlling the frequency of said oscillations comprising two lvacuum tubes each individually connected'in circuit with one of said reactance elements, the connections 4between each tube and its associated reactance element being such that similar rcontrol of said .tubes produces equal and opposite effects on the frequency of said oscillations, and means for simultaneously controlling said tubes in equal and opposite senses to produce a cumulative shift of the frequency of said oscillations.v 29. In combination, an oscillation generator for generating oscillations, means for controlling the frequency'of said oscillations comprising two vacuum tubes so connected in circuit with said generator that similar control of said tubes p roduces equal and oppositeeifects on the frequency of said oscillationsfand'means for 4selectively controlling said tubes in the same sense, selectivelylto shift in opposite senses the frequency of saidoscillations. y r y v HAROLD A.4 WHEEIER.

y DISOLAIM ER 2,099,156.-Harold A. Wheeler, Great Neck, N. Y. AUToMA'rIc FREQUENCY CON- T1201',v FOR OscILLATIoN SYSTEMS.

Patent dated November 16, 1937; Dis# a claimer filed September 10, 1938, by the patentes; the assignee, 'Hazeltine Corporation, assenting. Hereby jmlal `Gazette October 11, 1938.]

enters this disclaimer to claims 1, 2, and 3 of said patent.

y respectively, across at least a 4portion of saidv effective inductance or capacitance of said circuit lwhen said variable bias voltages exceed a predetermined value sumcient to overcome the cut-oil effect of saidv tlxed bias voltage.

26.` In combination, an oscillation generator including a frequency-determining circuit hav-v ing an inductive arm and al capacitive arm 4 eiectively in parallel, and means for controlling the oscillation frequency thereof comprising:y a pair o! conductive paths individually connected.

inductive armand across, at least a portion of said capacitive arm` of said circuit: and means 'dependent on the oscillation frequency of said generator for selectively controlling the resistance of either of said paths in the same senses thereby selectively to alter `in the opposite senses the effective inductance or capacitance oi said circuit. v

27. In combination, an oscillation generator for generating oscillations, means ior controlling the frequency of said oscillations comprising two vacuum tubes so connected in circuit with said. generator that similar control oi said tubes pro-v duces equal and opposite enects on the frequency oi said oscillations. and means for simultaneously controlling said tubes in equal and opposite senses `to produce a cumulative 'shift of the frequency of said-oscillations.

28. In combination, .an oscillation generator including a plurality of reactance elements for generating oscillations and means for controlling the frequency of said oscillations comprising two lvacuum tubes each individually connected'in circuit with one of said reactance elements, the connections 4between each tube and its associated reactance element being such that similar rcontrol of said .tubes produces equal and opposite effects on the frequency of said oscillations, and means for simultaneously controlling said tubes in equal and opposite senses to produce a cumulative shift of the frequency of said oscillations.v 29. In combination, an oscillation generator for generating oscillations, means for controlling the frequency'of said oscillations comprising two vacuum tubes so connected in circuit with said generator that similar control of said tubes p roduces equal and oppositeeifects on the frequency of said oscillationsfand'means for 4selectively controlling said tubes in the same sense, selectivelylto shift in opposite senses the frequency of saidoscillations. y r y v HAROLD A.4 WHEEIER.

y DISOLAIM ER 2,099,156.-Harold A. Wheeler, Great Neck, N. Y. AUToMA'rIc FREQUENCY CON- T1201',v FOR OscILLATIoN SYSTEMS.

Patent dated November 16, 1937; Dis# a claimer filed September 10, 1938, by the patentes; the assignee, 'Hazeltine Corporation, assenting. Hereby jmlal `Gazette October 11, 1938.]

enters this disclaimer to claims 1, 2, and 3 of said patent. 

